Moving from CCFT- to LED-backlit LCD modules

Many manufacturers of LCD modules are moving to LED-backlit units, and Sharp is responding to increasing customer demand for LED-backlit modules. LED backlights offer several advantages over CCFTs (Cold Cathode Fluorescent Tubes), with the primary reason being power savings, followed by the absence of mercury and superior low-temperature operation.

Moving a product from a CCFT-backlit module to an LED-backlit module raises a number of concerns for the designer:

The advantages of LED backlighting are many: better cold, shock and vibration tolerance, better color, less power consumption, more environmentally-friendly, and complete lack of RF output created by an inverter. ?However, as our world goes “greener,” more and more CCFT backlight subassembly suppliers are closing shop or restricting product output due to environ- mental concerns. This means a fundamental shift for designers and customers who are currently purchasing or considering CCFT-backlit LCD modules. ?Longevity of the backlight luminaire is often considered when contemplating the transition to LED, and the extreme ends of the temperature specification had been an area of concern.

The rule-of-thumb used to be, “Hot - CCFT, Cold - LED,” but no longer. ?CCFTs have long been the choice when a module is being operated at its maximum operating temperature, yet their lifetime is severely shortened when operating them near their minimum operating temperature. In some cases of continued operation in extreme cold, CCFTs may not reach even the greater half of their rated lifetime. LEDs tend to perform best in the opposite manner: long-lived when being operated at the module's minimum operating temperature, and shorter-lived when operating near the module's maximum operating temperature.

Meanwhile, by testing modules to the maximum limit of their specifications, Sharp guarantees their modules will perform to specifications at those published extremes. So even though the target application may involve higher ambient temperatures, as long as the design maintains the module within its published Absolute Maximum Values, you can have confidence that a Sharp module will perform to its lifetime specifications. Often the LED-backlit upgrade module will have the same Absolute Maximum Values specified.

When it comes to driver circuits for the two different backlight types, LED drivers again are at an advantage. CCFT drivers must generate a high 'start-up' (or striking) voltage and then maintain a certain high level of run voltage at a given current level. These high voltages require proper management and precautions in the final design; sealing against high dust and humidity levels, for instance.

In the past, the LED’s shortened lifetime at higher temperatures was a reason to reject an LED-backlit module for a high-temperature application. However, LED technology continues to evolve, allowing them to become better-suited for higher temperature applications.

Modern LED backlights feature higher-efficacy emitters, meaning in a watts-per-lumen sense they are not being operated nearly as close to the upper end of their performance envelope as in times past. They also feature adequate heatsinking so that all generated heat is dissipated properly. In Figure 1, the LED string and circuit board is bonded to the back chassis of the LCD module. This gives the LED string more than adequate heatsinking.

Figure 1: Close-up of LED backlight assembly

LED drivers are much less complex in nature, with no high voltages to generate and manage in the design. The LED string(s) do however require a constant-current supply with current limiting to prevent thermal runaway. Some suppliers are now making a’ driver-in-a-chip’ solution so driving an LED backlight is becoming even simpler.

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http://www.eetimes.com/design/smart-energy-design/4371743/LED-Focus